1. Field of the Invention
The present invention relates to an optical apparatus and an optical amplifier for use with an optical communications system, and more specifically to an optical apparatus provided with an optical amplification capability for compensation for a loss in a loss medium and a preferred optical amplifier for loss compensation.
2. Description of the Related Art
An optical amplifier is one of key components for realizing a long-distance and large-capacity optical communications system. Generally, an optical amplifier can be either a laser amplifier using stimulated emission from a population inversion medium or an amplifier based on a nonlinear optical effect such as Raman scattering, Brillouin scattering, etc. A laser amplifier can be an optical fiber amplifier using a rare earth element doped fiber or a semiconductor laser amplifier using a semiconductor optical amplification medium. The former is operated as an optical amplifier by optical pumping and the latter is operated as an optical amplifier by injected current pumping. In these optical amplifiers, a rare earth element doped fiber amplifier has a large merit in performance for a high gain, low noise, a broad band, a low coupling loss, low polarization dependency, high efficiency. In rare earth element doped fiber amplifiers, an erbium doped fiber optical amplifier (hereinafter referred to as an EDFA) is commonly used, and is practically used in the optical fiber communications system.
FIG. 1 shows an example of a configuration of an optical circuit of a common EDFA.
As shown in FIG. 1, basic parts of a common semiconductor optical amplification medium includes an erbium doped fiber (hereinafter referred to as an EDF) 100, a pumping light source 101, a wavelength multiplexer 102, a branch unit 103, an optical receiver 104, an optical filter 105 for cutting off noise light other than signal light, and an isolator 106 having the characteristic of passing light in only one direction.
The types of various devices forming an optical amplifier can be a fiber type, a discrete type (spatial coupling type), a waveguide type (PLC (planer lightwave circuit) type), etc. A fiber type device is featured by a low loss in coupling with fiber. However, since devices are coupled commonly by fusion splicing, size adjusting process is to be performed. Although a discrete type device can be downsized, it has a larger coupling loss with fiber than a fiber type device, and requires high spatial coupling precision. Therefore, it is not appropriate for integration when a large number of configuration parts are used. A waveguide type device can be easily integrated, and can be more easily downsized than a discrete type device. However, it has problems in characteristic such as a coupling loss with fiber, a polarization dependent loss (hereinafter referred to as a PDL), crosstalk, etc. The type of each of various devices and their configuring methods are selected from the characteristics, the cost, and the size of the entire optical amplifier. For example, the optical amplifier as shown in FIG. 1 is configured by performing fusion splicing on each component moduled with the fiber connected to an input port and an output port.
When the above-mentioned optical amplifier is downsized, for example, in the patent literature 1, the optical parts are configured as a waveguide type (PLC type) devices, and are collectively stored. In the patent literature 2, the optical parts other than the EDF are collectively stored by configuring a backpumping optical amplifier. Furthermore, an optical amplifier using a waveguide type optical amplification medium has been reported. The waveguide type optical amplification medium is inferior to the EDF in gain and noise figure (hereinafter referred to as an NF), efficiency, etc., but can be 10 cm in length, which is shorter than a fiber type amplification medium which is normally 10 cm long.
As described above, most of the conventional optical amplifiers are downsized for a single one-input-to-one-output optical amplifier, and there are few reports about downsizing into a plural-input-to-plural-output optical amplifier by integrating a number of optical amplifiers into one system because it seems that the system is a point-to-point system in a trunk type long-distance and large-capacity optical communications system for an optical amplifier of a conventional applicable area, and the number of available optical amplifiers is relatively small.
However, as the Internet has become widespread recently, the demand for communications traffic has exploded. To support the demand, the wavelength division multiplexing (hereinafter referred to as WDM) transmitting technology for communications by superposing a wave-multiplexed optical signal on an optical fiber has been in the limelight. Mainly a point-to-point system for connection of two points has been put to practical use, and the demand for further enlargement of the wavelength range has been issued. With the DWSM system introduced, a number of optical amplifiers for at least signal wavelengths are required for an optical amplifier for one wave in a transmitter/receiver, and correspondingly the optical amplifier is more earnestly requested to be downsized at a lower cost.
As an optical wave network system, etc., the construction of a more flexible and economical transport network is requested beyond the range of a long-distance and large-capacity trunk type system. For example, a photonic system for performing flexible optical routing such as an optical add/drop multiplexer (hereinafter referred to as an OADM) for branching or inserting a fixed wavelength during a transmitting node, an optical cross-connect (hereinafter referred to as an OXC) for switching a path, and performing editing and fault recovering processes, etc. is required. The optical network system necessarily uses a number of optical parts such as an optical switch, a variable attenuator, an OADM, etc. When optical parts are used, a number of optical amplifiers for loss compensation for various optical parts are required. Practically, several hundred levels of optical amplifiers are expected in on OXC device. These systems are expected to be led to the vicinity of a metropolis, and further to an office or home, thereby further enhancing the demand for a small optical amplifier.
In a higher-speed optical communications system such as a 40 Gb/s system, etc. a dispersion compensator, a polarization mode dispersion (hereinafter referred to as a PMD) compensator, etc. are required. Especially, a PMD compensator is to be provided for each signal channel. In this optical communications system, a number of small optical amplifiers for compensating for a loss generated in the optical parts such as the above-mentioned PMD compensator are required.
As for the optical amplifier for loss compensation for various optical parts, for example, an optical element without a loss, etc. disclosed by National Publication of International Patent Application No. 02334 is well known. It provides an optical element without a loss by providing a single optical amplifier in the remote pumping method on the input side of a single optical element, and incorporating the single optical amplifier into the single optical element. However, since this well-known technology does not provide a practical configuration appropriate for incorporating an optical amplifier to be downsized into a loss medium, it does not satisfy the request to a small optical amplifier for loss compensation as described above.
As for the request to downsize an optical amplifier, a plurality of optical amplifiers can be incorporated into one device so as to downsize each unit of optical amplifier (for example, EDWA Array™, etc. of Teem Photonics, etc.). The optical amplifier practically uses an erbium doped waveguide (hereinafter referred to as an EDW) as an amplification medium to downsize the entire system by incorporating a plurality of optical amplifiers in parallel. However, a pumping light source and an isolator are external devices. Nevertheless, the above-mentioned optical amplifier aims at only downsizing each optical amplifier basically by integration, but a practical configuration of an optical amplifier appropriate for integration with a loss medium is not realized. Therefore, the request for a small optical amplifier for loss compensation is not completely satisfied.
Patent Literature 1
Japanese Patent Application Laid-open No. Hei 10-107350
Patent Literature 2
Japanese Patent Application Laid-open No. Hei 3-296025